A number of venting valve devices for die-casting, e.g. according to German Patent No. 4 302 798, are actuated by the action of the metal, the device comprising a force transducer which is actuated by the cast material, and the transducer being mechanically connected to the movable shutter of the venting valve. Indeed, the attained shut-off speeds are very fast, thus allowing an application in high-speed die-casting as well. The disadvantage is that due to the necessary control stroke of the force transducer, liquid metal enters the guide bore of the latter at each injection stroke. This leads to an increased wear of the control piston and of its guide bore. These locations may also be subject to cold pickups which may block the control pistons. Even if this control function is only slightly impaired, the shutting action of the evacuation piston will be too slow, and the liquid metal will enter the venting valve, solidify, and block the entire installation. This leads to increased standstill and maintenance costs of the machine. Moreover, valve devices of this kind have many mechanical parts, and the opening force acting in the venting operation must be small in order not to counteract a fast shut-off time.
It has been attempted to eliminate the described drawbacks by closing the control member by an electric control circuit instead of its actuation by the cast material. German Publication No. 3 912 006 discloses a gas evacuation device in a high-speed die-casting installation where a gas evacuation valve is closed by a combination of a control circuit and a valve driving unit. This device, comprises a relatively complex control circuit, the proper control valves being pilot controlled by servo valves in order to attain short switching times. In the process, the actual control signals have to be amplified, converted, and adapted to the existing interfaces.
Finally, German Publication No. 4 216 773 discloses a die-casting installation whose releasing device comprises a metal sensor engaging in the casting path which is followed by a switching device, the valve being closed with a delay.